Method of manufacturing a p-AlGaN layer

ABSTRACT

The method according to the present invention includes a first step of supplying the Group V source gas at a flow rate B 1  (0&lt;B 1 ) and supplying the gas containing magnesium at a flow rate C 1  (0&lt;C 1 ) while supplying the Group III source gas at a flow rate A 1  (0≦A 1 ); and a second step of supplying a Group V source gas at a flow rate B 2  (0&lt;B 2 ) and supplying a gas containing magnesium at a flow rate C 2  (0&lt;C 2 ) while supplying a Group III source gas at a flow rate A 2  (0&lt;A 2 ). The first step and the second step are repeated a plurality of times to form a p-Al x Ga 1-x N (0≦x&lt;1) layer, and the flow rate A 1  is a flow rate which allows no p-Al x Ga 1-x N layer to grow and satisfies A 1 ≦0.5 A 2 .

TECHNICAL FIELD

The present invention relates to a p-AlGaN layer, and in particular to amagnesium-doped p-AlGaN layer having constant aluminum compositionratio. The present invention also relates to a method of manufacturingthe same, and a Group III nitride semiconductor light emitting deviceusing the same.

RELATED ART

In recent years, ultraviolet LEDs using Group III nitride semiconductordevices are actively researched and developed since they are expected tobe used for back lights of liquid crystal displays, excitation lightsources of white LEDs for lighting and sterilization, and medical uses,etc.

In general, the conductivity type of semiconductors is determineddepending on the kind of impurities added. By way of example, when anAlGaN material is made to have p-type conductivity, magnesium istypically used as an impurity. On this occasion, the magnesium addedserves as acceptors, and holes in this AlGaN material serve as carriers.

However, when a semiconductor layer is thus formed by MOCVD (metalorganic chemical vapor deposition) using magnesium as an impurity, aphenomenon called “doping delay” in which impurities are notsufficiently introduced into the semiconductor layer in growth wouldoccur.

One of the reasons for this is that magnesium to be supplied to thesemiconductor layer would adhere to inner walls and the like of a growthsystem and pipes in an initial stage of the growth of the semiconductorlayer and it would not be supplied sufficiently to the semiconductorlayer accordingly.

On the other hand, Patent Document 1 discloses a technique of preventingdoping delay by supplying a magnesium-containing gas into a growthsystem prior to the formation of the semiconductor layer so that theamount of the above-described adherence is saturated.

Beside such doping delay in an initial growth stage of a semiconductorlayer, doping delay is also known to occur after the initial growthstage. One of the reasons is for example as follows. For example,hydrogen atoms generated when a gas supplied into the semiconductorlayer in growth is partially introduced into crystals are bound tonitrogen atoms in the crystals by hydrogen-bonding to release electrons.Meanwhile, holes are released from magnesium atoms which are p-typeimpurities disposed at lattice arrangements where gallium atoms shouldoriginally be disposed. The released electrons and the released holesare combined to electrically compensate one another, which consequentlyprevents magnesium added for achieving p-type conductivity from servingas acceptors. This leads to decline in the carrier concentration in thesemiconductor layer.

Further, shorter wavelength ultraviolet LEDs increase demand forAl_(x)Ga_(1-x)N materials having a high aluminum composition ratio and awide band gap for use in an active layer. A high aluminum compositionratio x increases ionization energy of magnesium itself; therefore, ithas been difficult to achieve high carrier concentration.

Such decline in the carrier concentration increases resistance, and thiscauses heat generation or the like, which makes it impossible to obtainsufficient light output.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] JP2007-42886 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the invention is to solve the above problems and to providea p-AlGaN layer achieving improved carrier concentration and lightoutput, a method of manufacturing the same, and a Group III nitridesemiconductor light emitting device using the same.

Means for Solving the Problem

In order to achieve the above object, the present invention primarilyincludes the following components.

(1) A method of manufacturing a p-AlGaN layer, the p-AlGaN layer beingone p-Al_(x)Ga_(1-x)N layer (0≦x<1) doped with magnesium, which isformed by MOCVD, comprising the steps of: a first step of supplying aGroup V source gas at a Group V source gas flow rate B₁ (0<B₁) andsupplying a gas containing magnesium at a Mg-containing gas flow rate C₁(0<C₁) while supplying a Group III source gas at a Group III source gasflow rate A₁ (0≦A₁); and a second step of supplying a Group V source gasat a Group V source gas flow rate B₂ (0<B₂) and supplying a gascontaining magnesium at a Mg-containing gas flow rate C₂ (0<C₂) whilesupplying a Group III source gas at a Group III source gas flow rate A₂(0<A₂), wherein the first step and the second step are repeated aplurality of times to form the p-Al_(x)Ga_(1-x)N layer, and the GroupIII source gas flow rate A₁ is a flow rate which allows nop-Al_(x)Ga_(1-x)N layer to grow and satisfies A₁≦0.5A₂.

(2) A method of manufacturing a p-AlGaN layer, the p-AlGaN layer beingone p-Al_(x)Ga_(1-x)N layer (0≦x<1) doped with magnesium, which isformed by MOCVD, comprising the steps of: a first step of supplying aGroup V source gas at a Group V source gas flow rate B₁ (0<B₁) andsupplying a gas containing magnesium at a Mg-containing gas flow rate C₁(0<C₁) while supplying a Group III source gas at a Group III source gasflow rate A₃ (0<A₃); and a second step of supplying a Group V source gasat a Group V source gas flow rate B₂ (0<B₂) and supplying a gascontaining magnesium at a Mg-containing gas flow rate C₂ (0<C₂) whilesupplying a Group III source gas at a Group III source gas flow rate A₂(0<A₂), wherein the first step and the second step are performed to formthe p-Al_(x)Ga_(1-x)N layer, and the Group III source gas flow rate A₃is a flow rate which allows only initial growth nuclei of thep-Al_(x)Ga_(1-x)N layer to grow and satisfies A₃≦0.5A₂.

(3) A method of manufacturing a p-AlGaN layer, the p-AlGaN layer beingone p-Al_(x)Ga_(1-x)N layer (0≦x1) doped with magnesium, which is formedby MOCVD, comprising the steps of: a first step of supplying a Group Vsource gas at a Group V source gas flow rate B₁ (0<B₁) and supplying agas containing magnesium at a Mg-containing gas flow rate C₁ (0<C₁)while supplying a Group III source gas at a Group III source gas flowrate A₃ (0<A₃); and a second step of supplying a Group V source gas at aGroup V source gas flow rate B₂ (0<B₂) and supplying a gas containingmagnesium at a Mg-containing gas flow rate C₂ (0<C₂) while supplying aGroup III source gas at a Group III source gas flow rate A₂ (0<A₂),wherein the first step and the second step are repeated a plurality oftimes to form the p-Al_(x)Ga_(1-x)N layer, and the Group III source gasflow rate A₃ is a flow rate which allows only initial growth nuclei ofthe p-Al_(x)Ga_(1-x)N layer to grow and satisfies A₃≦0.5A₂.

(4) The method of manufacturing a p-AlGaN layer according to any one of(1) to (3) above, wherein the Group V source gas flow rate B₁ in thefirst step is equal to the Group V source gas flow rate B₂ in the secondstep, and/or the Mg-containing gas flow rate C₁ in the first step isequal to the Mg-containing gas flow rate C₂ in the second step.

(5) The method of manufacturing a p-AlGaN layer according to any one of(1) to (4) above, wherein when a relationship between a Group III sourcegas flow rate and a crystal growth rate is evaluated from the crystalgrowth rate in the second step, the Group III source gas flow rate inthe first step is a flow rate such that a growth rate of thep-Al_(x)Ga_(1-x)N layer corresponding to the flow rate is 0.03 nm/s orless.

(6) The method of manufacturing a p-AlGaN layer according to any one of(1) to (5) above, wherein the aluminum composition ratio x of thep-Al_(x)Ga_(1-x)N layer is in the range of 0 to 0.8.

(7) A Group III nitride semiconductor light emitting device comprising ap-Al_(x)Ga_(1-x)N layer manufactured by the method of manufacturing ap-AlGaN layer according to any one of (1) to (6) above.

(8) A p-AlGaN layer doped with magnesium, which has an aluminumcomposition ratio x of 0.2 or more and less than 0.3 and a carrierconcentration of 5×10¹⁷/cm³ or more.

(9) A p-AlGaN layer doped with magnesium, which has an aluminumcomposition ratio x of 0.3 or more and less than 0.4 and a carrierconcentration of 3.5×10¹⁷/cm³ or more.

(10) A p-AlGaN layer doped with magnesium, which has an aluminumcomposition ratio x of 0.4 or more and less than 0.5 and a carrierconcentration of 2.5×10¹⁷/cm³ or more.

(11) A Group III nitride semiconductor light emitting device comprisingthe p-Al_(x)Ga_(1-x)N layer according to any one of (8) to (10) above.

Effect of the Invention

The present invention can provide a p-AlGaN layer having a carrierconcentration and a light output which are improved by forming onep-AlGaN layer doped with magnesium using MOCVD under conditions where aGroup III source gas is supplied in a first step at a flow rate of 0 orat a flow rate equal to or less than a flow rate of a Group III sourcegas supplied in a second step. The present invention can also provide amethod of manufacturing the same and a Group III nitride semiconductorlight emitting device.

Further, the present invention can provide a p-AlGaN layer achieving acarrier concentration and a light output which are improved by repeatingthe first step and the second step a plurality of times, a method ofmanufacturing the same, and a Group III nitride semiconductor lightemitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an example of an MOCVD systemfor manufacturing a p-AlGaN layer in accordance with the presentinvention.

FIG. 2 is a schematic cross-sectional view illustrating an example of agrowth furnace in an MOCVD system for manufacturing a p-AlGaN layer inaccordance with the present invention.

FIG. 3 shows XRD diffraction patterns of p-Al_(0.23)Ga_(0.77)N layers inaccordance with a method of the present invention and a conventionalmethod.

FIGS. 4( a) and 4(b) show TEM images of p-Al_(0.23)Ga_(0.77)N layers inaccordance with a method of the present invention and a conventionalmethod, respectively.

FIGS. 5( a) and 5(b) show differential interference contrast micrographsof the outermost surfaces of p-Al_(0.23)Ga_(0.77)N layers in accordancewith a method of the present invention and a conventional method,respectively.

FIG. 6 is a schematic cross-sectional view illustrating a Group IIInitride semiconductor light emitting device in accordance with thepresent invention.

FIG. 7 shows a SIMS profile of a p-Al_(0.36)Ga_(0.64)N layer in a lightemitting device of Example 9.

FIG. 8 shows a SIMS profile of a p-Al_(0.36)Ga_(0.64) layer in a lightemitting device of Comparative Example 6.

FIG. 9 is a graph showing collected carrier concentrations calculatedfrom specific resistance values of p-Al_(x)Ga_(1-x)N layers inaccordance with a method of the present invention and a conventionalmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, embodiments of a method of manufacturing a p-AlGaN layer inaccordance with the present invention will be described with referenceto the drawings. FIG. 1 is a schematic cross-sectional view illustratingan example of an MOCVD system for manufacturing a p-AlGaN layer inaccordance with the present invention. This MOCVD system 100 includes areaction furnace 103 having a first gas supply port 101 and a second gassupply port 102. The first gas supply port 101 supplies a carrier gassuch as hydrogen gas 104 and/or nitrogen gas 105, a Group III source gassuch as TMA (trimethylaluminium) 106 and TMG (trimethylgallium) 107, amagnesium-containing gas 108 as an impurity source gas, and/or the liketo the reaction furnace 103. Meanwhile, the second gas supply port 102supplies a carrier gas such as hydrogen gas 104 and/or nitrogen gas 105,and a Group V source gas 109 such as ammonia to the reaction furnace103.

With respect to a method of manufacturing a p-AlGaN layer in accordancewith the present invention, magnesium-doped p-Al_(x)Ga_(1-x)N (0≦x<1)having constant aluminum composition ratio x is formed using such anMOCVD system 100 described above by a first step of supplying a Group Vsource gas at a Group V source gas flow rate B₁ (0<B₁) and supplying agas containing magnesium at a Mg-containing gas flow rate C₁ (0<C₁)while supplying a Group III source gas at a Group III source gas flowrate A₁ (0≦A₁); and a second step of supplying a Group V source gas at aGroup V source gas flow rate B₂ (0<B₂) and supplying a gas containingmagnesium at a Mg-containing gas flow rate C₂ (0<C₂) while supplying aGroup III source gas at a Group III source gas flow rate A₂ (0<A₂). Thefirst step and the second step are repeated a plurality of times to formthe p-Al_(x)Ga_(1-x)N layer, and the Group III source gas flow rate A₁is a flow rate which allows no p-Al_(x)Ga_(1-x)N layer to grow andsatisfies A₁≦0.5A₂. Thus, the carrier concentration and light output ofthe p-AlGaN layer can be improved.

Here, cases where “the Group III source gas flow rate A₁ is a flow ratewhich allows no p-AlGaN layer to grow” mean cases where “the thicknessof p-AlGaN is not enough to form a substantial layer” to include suchcases where no p-AlGaN is grown, or where an initial crystal nucleus ofp-AlGaN (for example, an island-like crystal) is grown but the thicknessis not enough to form a substantial layer. Specifically, the caseinclude a case where A₁ is a flow rate which allows no p-AlGaN to grow(at least the cases where A₁=0 apply to these) and cases where A₁ is aflow rate which allows only initial growth nuclei of p-AlGaN to grow (atleast the cases where A₁>0 apply to these). Such a Group III source gasflow rate A₁ satisfies at least the relation 0≦A₁≦0.5A₂. Further, thefirst step serves to maintain the state where no layer is grown for anintended period. On this occasion, the Group V source gas flow rate B₁and the Mg-containing gas flow rate C₁ are preferably equal to or morethan a flow rate which allows a layer to grow as long as the Group IIIsource gas is supplied. In other words, B₁≧B₂ and C₁≧C₂ are preferablysatisfied. This is to prevent nitrogen leakage and supply enough Mg intothe system while the layer growth is interrupted.

Further, when two kinds of gases, TMA (trimethylaluminium) and TMG(trimethylgallium) for example, are supplied as Group III source gases,the Group III source gas flow rate A₁ represents the total flow rate ofthese gases.

Note that “the aluminum composition ratio is constant” means that thealuminum composition ratio x of the layer which is grown in each of thesecond steps does not change irrespective of the repeat count of thefirst step and the second step. Specifically, this means that the gasflow rate A₁ in each repetition is an equal flow rate. However, inmeasuring the aluminum quantity by SIMS, in terms of the analysisprinciple, the aluminum composition ratio varies in the depth direction.Further, the aluminum composition ratio may be fluctuated in the layeror aluminum may be distributed in the plane, due to the system inepitaxial growth. Such phenomena can be accepted because they are causedalso in conventional methods. Note that the aluminum composition in thepresent invention is a value measured at the substrate center.

FIG. 3 is an x-ray diffraction (XRD) image of a p-Al_(0.23)Ga_(0.77)Nlayer (supply of the Group III source gas is modulated: mode of thepresent invention) manufactured by a method in accordance with thepresent invention and a p-Al_(0.23)Ga_(0.77)N layer (Group III sourcegas without supply modulation: conventional mode) manufactured by aconventional method. Table 1 shows representative values of XRDintensity corresponding to Miller indices [002] and [102] which provideindications of crystal quality. The former represents “tilt” withrespect to the growth axis direction of an initial growth nucleus, whilethe latter represents the degree of “twist” with respect to the growthin-plane direction. FIGS. 4( a) and 4(b) show transmission electronmicroscope (TEM) images of p-Al_(0.23)Ga_(0.77)N layers in accordancewith a method of the present invention and a conventional method,respectively. FIGS. 5( a) and 5(b) show electron diffraction patterns ofthe outermost surfaces of p-Al_(0.23)Ga_(0.77)N layers in accordancewith a method of the present invention and a conventional method,respectively.

TABLE 1 XRD [002] [102] Invention mode 195 456 Conventional mode 234 442

As shown in FIG. 3 and FIGS. 4( a) and 4(b), a method of the presentinvention is equivalent to a conventional method in macroscopic XRDspectra, and no periodic perturbation in crystal growth was observed inthe microscopic TEM images and the electron diffraction patterns. Thus,only one single crystal layer is found to be grown in either case. Notethat while XRD spectra of FIG. 3 have two peaks caused by components ofdifferent axes at approximately 75° in the conventional mode, thesepeaks are lost in the method of the present invention. Further, therepresentative value corresponding to Miller index [002] shown in Table1 is reduced. Thus, the present invention is found to contribute to theimprovement in crystallinity. Note that Table 1 shows that the component“twist” with respect to the growth in-plane direction of the initialgrowth nucleus differs little, while the “tilt” with respect to thegrowth axis direction is reduced. This suggests that the components ofdifferent axes are reduced, and the orientation of the initial growthnuclei in the growth direction is improved.

A method of manufacturing a p-AlGaN layer in accordance with the presentinvention will be described. First, as shown in FIG. 2, a base substrate111 is placed on a susceptor 110 in the reaction furnace 103. Examplesof the base substrate 111 include a GaN substrate, a sapphire substrate,and an AlN template substrate in which an AlN layer is provided on asapphire substrate. Alternatively, such a substrate on which asemiconductor layer is stacked may be used.

Next, in a first step, a carrier gas such as hydrogen gas 104 and/ornitrogen gas 105 and a Group V source gas 109 such as ammonia aresupplied from the second gas supply port 102 into this reaction furnace103. Further, a Group III source gas is supplied from the first gassupply port 101 into this reaction furnace 103 at a flow rate which doesnot result in layer growth or at a flow rate which causes only initialnucleus growth. Along with these source gases, a magnesium-containinggas 108 is supplied. The Group V source gas 109 here is supplied tocontrol decline in partial pressure of nitrogen in the reaction furnace103 and to protect the outer most surface where crystal growth occurs.Note that CP₂Mg (bis-cyclopentadienyl magnesium) or the like can be usedas the magnesium-containing gas 108.

After a predetermined time, in a second step, a Group III source gas issupplied from the first gas supply port 101 at a flow rate which resultsin layer growth. Along with this source gas, a magnesium-containing gas108 is supplied. Concurrently, a Group V source gas 109 is supplied fromthe second gas supply port 102 at a flow rate which results in layergrowth. Note that the above “predetermined period of time” formaintaining the first step is preferably about 5 seconds or more and 60seconds or less. When the predetermined period is too short, the effectsof the present invention cannot be obtained sufficiently. Otherwise whenthe predetermined time is too long, Mg is introduced excessively, whichwould make Mg cause defects to deteriorate the crystallinity or reducecarrier concentration in subsequent crystal growth.

In a method of manufacturing a p-AlGaN layer in accordance with thepresent invention, the p-Al_(x)Ga_(1-x)N layer (0≦x<1) doped withmagnesium is formed using MOCVD by a first step of supplying a Group Vsource gas at a Group V source gas flow rate B₁ (0<B₁) and supplying agas containing magnesium at a Mg-containing gas flow rate C₁ (0<C₁)while supplying a Group III source gas at a Group III source gas flowrate A₃ (0<A₃); and a second step of supplying a Group V source gas at aGroup V source gas flow rate B₂ (0<B₂) and supplying a gas containingmagnesium at a Mg-containing gas flow rate C₂ (0<C₂) while supplying aGroup III source gas at a Group III source gas flow rate A₂ (0<A₂). Thefirst step and the second step are performed to form thep-Al_(x)Ga_(1-x)N layer, and the Group III source gas flow rate A₃ is aflow rate which allows only initial growth nuclei of thep-Al_(x)Ga_(1-x)N layer to grow and satisfies A₃≦0.5A₂. With such amethod, surfaces of the reaction furnace 103 and the pipes and the likecan be previously coated with adequate magnesium. This makes it possibleto suppress reduction in the magnesium concentration of the AlGaN layerin initial growth, namely, doping delay. Here, the flow rate whichallows only initial nuclei to grow refers to a flow rate which leads toa state where for example, island-like initial crystal nuclei are formedbut the thickness is not enough to form a substantial layer. Such aGroup III source gas flow rate A₃ satisfies at least the relation0<A₃≦0.5A₂. Note that in the present invention, the growth of onlyinitial growth nuclei can be confirmed by observing the surface of asubstrate of which growth is interrupted after the first step with theuse of a metallurgical microscope or a SEM to find island-like initialgrowth nuclei dispersed on the substrate surface.

Further, when two kinds of gases, TMA (trimethylaluminium) and TMG(trimethylgallium) for example, are supplied as Group III source gases,the Group III source gas flow rate A₃ represents the total flow rate ofthese gases.

The Group V source gas flow rate B₁ in the first step is preferablyequal to the Group V source gas flow rate B₂ in the second step, and/orthe Mg-containing gas flow rate C₁ in the first step is preferably equalto the Mg-containing gas flow rate C₂ in the second step. That is, it ispreferable that the flow rate A₁ or A₃ of the Group III source gas inthe first step is different from the flow rate A₂ of the Group IIIsource gas in the second step while the Group V source gas flow rate isconstant.

The adherence of magnesium to the surface of the AlGaN layer in growthmakes predominant the growth in lateral directions, and reduces thecrystal growth rate in the growth axis direction. This increases thefrequency of regional growth of initial nuclei (three-dimensional);thus, the effective surface area is increased, and the frequency ofmagnesium introduction is improved by suppressing the migration ofatoms. Therefore, forcible introduction of magnesium by the physicaladherence, and improvement in the frequency of magnesium introductiondue to the reduction in the growth rate improve the magnesiumconcentration of the AlGaN layer.

Further, since this effect is temporary, the above first step and thesecond step are repeated a plurality of times, so that the magnesiumconcentration of the AlGaN layer can be maintained at a constant highconcentration. For example, even when a p-AlGaN layer having thealuminum composition ratio of 0.15 or more, which reduces the magnesiumconcentration due to the increase of the ionization energy of magnesiumitself is formed, a p-AlGaN layer having higher magnesium concentrationthan conventional can be manufactured.

Furthermore, in a method of manufacturing a p-AlGaN layer in accordancewith the present invention, one p-AlGaN layer doped with magnesium isformed particularly using the above-mentioned MOCVD system 100 bypreviously performing a step of supplying a Group III source gas at aflow rate reduced to a level that allows only initial nuclei to grow andsupplying a Group V source gas and a gas containing magnesium before thestep of supplying a gas containing magnesium along with the Group IIIand Group V source gases at flow rates which allow crystals to grow.Thus, the magnesium doping level in the p-AlGaN layer can be maintained.

As initial nuclei have portions containing sufficient Mg, which havebeen forcibly formed in the initial growth, the source materials to besupplied later are predominantly diffused in lateral directions andtheir crystal growth rates in the growth axis direction are reducedaccordingly. In other words, the diffused molecules are introduced intostep ends at a higher rate, which promotes the formation of a flat layer(surfactant effect). However, this effect is temporary and initialgrowth nuclei causing irregularities begin to form again after the stepflow growth (growth in lateral directions) continued for a while. Thisinvolves increase in the surface area to suppress in-plane diffusion ofMg itself, and the introduction frequency of Mg into the layer isimproved, which consequently improves the magnesium concentration in theAlGaN layer.

Thus, according to the present invention, a step of supplying the GroupIII gas at a flow rate reduced to a level that allows only initialnuclei to grow and supplying the Group V source gas and the gascontaining magnesium is provided, so that introduction of Mg can beimproved and the crystallinity can be improved by growth in lateraldirections.

The Group III source gas flow rate (A₁ or A₃) in the first step isdifferent from the Group III source gas flow rate A₂ in the second step,and the Group III source gas flow rate in the first step is preferably ½or less, more preferably ¼ or less the Group III source gas flow rate A₂in the second step. In particular, the relationship between the GroupIII source gas flow rate and the crystal growth rate is evaluated fromthe thickness of a layer grown per unit time in a range where crystalgrowth can be observed (namely, crystal growth rate) (for example, therelationship between a plurality of pairs of Group III source gas flowrates and crystal growth rates in a flow rate range of 10 sccm to 30sccm is linearized). When the Group III source gas flow rate (A₁ or A₃)in the first step is extrapolated from this relationship, the flow rateis preferably such that the crystal growth rate of the p-Al_(x)Ga_(1-x)Nlayer that corresponds to the flow rate (A₁ or A₃) in the first step is0.03 nm/s or less, more preferably 0.01 nm/s to 0.03 nm/s based on thecomputation. Note that the figure of the Group III source gas flow ratein the first step and the second step (the ratio of Ga and Al) may notnecessarily show the multiple proportion relationship. Specifically, theAl composition of the initial growth nuclei created in the first stepmay not necessarily be the same as the Al composition of the initialgrowth nuclei created in the second step. This is for making the initialgrowth nuclei created in the first step contain Mg at a maximum and forimproving the crystallinity of a crystal film formed in the second step,thereby maximizing the effect of the present invention. Note thatalthough the Al compositions are different, the crystal layers obtainedin the mode of the present invention can be deemed to have constant Alcomposition because an initial growth nuclei created in the first stephas negligible thickness as compared to the crystal film formed in thesecond step. In addition, when the computational growth rate is 0.01nm/s to 0.03 nm/s, the Group III source material is less probable to bepresent in the substrate surface; for example, only island-like initialgrowth nuclei are created. Thus, the thickness does not increase enoughto form a substantial layer even over a long period of time. Note thatif the Group III source gas flow rate in the first step iscomputationally a flow rate such that the crystal growth rate is lessthan 0.01 nm/s, the decomposition of the initial growth nuclei becomespredominant over its growth. Thus, p-AlGaN is not grown.

The flow rate of the Group III source gas which allows only initialnuclei to grow cannot be specified definitely because it variesdepending on the shape, temperature, and the Group V source gas flowrate of the MOCVD system. However, the Group III source gas flow rate(A₁ or A₃) in the first step is preferably, for example, 1 sccm to 10sccm while the Group III source gas flow rate A₂ in the second step is20 sccm to 50 sccm. Further, the Group V source gas flow rates B₁ and B₂in the first step and the second step may be, for example, 5 slm to 50slm (standard liter per minute). Further, the Mg-containing gas flowrates C₁ and C₂ in the first step and the second step may be, forexample, 20 sccm to 200 sccm.

In either case where no Group III source gas is flown (A₁=0 sccm) orwhere the Group III source gas is flown to allow only initial nuclei togrow (A₁, A₃=1 sccm to 10 sccm) in the first step, the magnesiumconcentration of the AlGaN layer can be maintained at a high, constantconcentration by repeating the first step and the second step aplurality of times. However, it is more preferable to allow initialnuclei to grow because the effect of improving crystallinity can beachieved more easily.

A p-AlGaN layer having high magnesium concentration and improvedcrystallinity can be manufactured by the above methods of the presentinvention.

Further, the aluminum composition ratio of the p-AlGaN layer may be 0 to0.8 . Note that the aluminum composition ratio x can be found bymeasuring the emission wavelength of photoluminescence and convertingthe emission wavelength of photoluminescence using Bowing parametersdescribed in Yun F. et al, J. Appl. Phys. 92, 4837 (2002).

Subsequently, embodiments of a Group III nitride semiconductor lightemitting device of the present invention will be described withreference to the drawings. A Group III nitride semiconductor lightemitting device 200 in accordance with the present invention may have astructure including an AlN template substrate having an AlN strainbuffer layer 202 on a sapphire substrate 201; and a superlattice strainbuffer layer 203, an n-AlGaN layer 204, a light emitting layer 205, ap-AlGaN blocking layer 206, a p-AlGaN guide layer 207, a p-AlGaNcladding layer 208, and a p-GaN contact layer 209 on the AlN templatesubstrate. These p-AlGaN layers can be grown by the above methods ofmanufacturing a p-AlGaN layer in accordance with the present invention.

Further, according to the methods of manufacturing a p-AlGaN layer inaccordance with the present invention, a p-AlGaN layer having a carrierconcentration of 5×10¹⁷/cm³ or more and preferably 1×10¹⁸/cm³ or lesscan be obtained as a magnesium-doped p-Al_(x)Ga_(1-x)N layer having aconstant aluminum composition ratio when the aluminum composition ratiox is 0.2 or more and less than 0.3 . Further, when the aluminumcomposition ratio x is 0.3 or more and less than 0.4, a p-AlGaN layerhaving a carrier concentration of 3.5×10¹⁷/cm³ or more and preferably5×10¹⁷/cm³ or less can be obtained. Furthermore, when the aluminumcomposition ratio x is 0.4 or more and less than 0.5, a p-AlGaN layerhaving a carrier concentration of 2.5×10¹⁷/cm³ or more and preferably3.5×10¹⁷/cm³ or less can be obtained.

Note that FIGS. 1 to 6 show examples of representative alternativeembodiments, and the present invention is not limited to theseembodiments.

EXAMPLE Example 1

In Example 1 , after an AlN template substrate having a strain bufferlayer was placed in a growth furnace shown in FIG. 1 and FIG. 2 and thetemperature was increased to 1050° C. at 10 kPa, a first step and asecond step were alternately repeated 120 times. In each of the firststeps, while Group III source gases (TMG flow rate: 4 sccm, TMA flowrate: 5 sccm) were flown, a carrier gas (mixture of N₂ and H₂, flowrate: 50 slm), a Group V source gas (NH₃, flow rate: 15 slm), and aCP₂Mg gas (flow rate: 50 sccm) were supplied for 15 seconds (supply timet₁). In each of the subsequent second steps, only the flow rates of theGroup III source gases were changed to a TMG flow rate of 20 sccm and aTMA flow rate of 25 sccm, and the Group III source gases, the carriergas, the Group V source gas, and the CP₂Mg gas were supplied for 60seconds (supply time t₂). Thus, a p-Al_(0.23)Ga_(0.77)N layer having athickness of 1080 nm was formed. (Note that the unit “sccm” of the aboveflow rates expresses the amount (cm³) of gas flown per minute at 1 atm(atmospheric pressure: 1013 hPa) at 0° C.) Note that in the first step,initial growth nuclei were grown, but a layer was not grown. The crystalgrowth rate in the second step was 0.15 nm/s. The computational growthrate corresponding to the Group III source gas flow rate in the firststep was 0.03 nm/s.

Example 2

In Example 2, a p-Al_(0.23)Ga_(0.77)N layer having a thickness of 1080nm was formed by a similar method to Example 1 except for that thesupply time t₂ was 30 seconds, and the repeat count was 240.

Example 3

In Example 3, a p-Al_(0.23)Ga_(0.77)N layer having a thickness of 1080nm was formed by a similar method to Example 1 except for that thesupply time t₂ was 45 seconds, and the repeat count was 180.

Example 4

In Example 4, a p-Al_(0.23)Ga_(0.77)N layer having a thickness of 1080nm was formed by a similar method to Example 1 except for that thesupply time t₂ was 120 seconds, and the repeat count was 60.

Example 5

In Example 5, a p-Al_(0.23)Ga_(0.77)N layer having a thickness of 1080nm was formed by a similar method to Example 1 except for that thesupply time t₂ was 7200 seconds, and the repeat count was one.

Reference Example

In Reference Example, a p-Al_(0.23)Ga_(0.77)N layer having a thicknessof 1080 nm was formed by a similar method to Example 5 except for thatno Group III source gas was flown and no initial growth nucleus wasgrown in the first step.

Comparative Example 1

In Comparative Example 1, a p-Al_(0.23)Ga_(0.77)N layer having athickness of 1080 nm was formed by a similar method to Example 1 exceptfor that the supply time t₁ was 0 second, the supply time t₂ was 7200seconds, and the repeat count was one.

(Evaluation 1)

After each of the forgoing Examples 1 to 5, Reference Example, andComparative Example 1; annealing was performed at 800° C. for 5 minutesin a nitrogen atmosphere using a lamp annealing furnace. Then, thein-plane specific resistance of the p-AlGaN layers was measured using aneddy current sheet resistance measurement system (MODEL1318 manufacturedby Lehighton Electronics, inc). The results of evaluating carrierconcentrations calculated from the specific resistances under conditionswhere the activation depth is 0.5 μm and the mobility is 5 are shown inTable 2.

TABLE 2 Carrier Total concentration Supply Supply Thickness thicknesscalculated p-AlGaN Al time time Repeat per of single Specific fromSpecific single film composition t1 t2 count r repetition film layerresistance resistance layer ratio (second) (second) (number) (nm) (nm)(W × cm) (/cm3) Example 1 0.23 15 60 120 9 1080 1.7 7.35 ′ 1017 Example2 0.23 15 30 240 4.5 1080 2.19 5.70 ′ 1017 Example 3 0.23 15 45 180 61080 2 6.24 ′ 1017 Example 4 0.23 15 120 60 18 1080 2.67 4.67 ′ 1017Example 5 0.23 15 7200 1 1080 1080 2.7 4.63 ′ 1017 Reference 0.23 157200 1 1080 1080 2.73 4.58 ′ 1017 Example Comparative 0.23 0 7200 1 10801080 2.75 4.53 ′ 1017 Example 1

Table 2 shows that the specific resistances in Examples 1 to 5 werereduced as compared with Comparative Example 1, therefore Examples 1 to5 in accordance with the present invention have an effect of increasingcarrier concentration as compared with Comparative Example 1.

Example 6

In Example 6, after an AlN template substrate having a strain bufferlayer was placed in a growth furnace shown in FIG. 1 and FIG. 2 and thetemperature was increased to 1050° C. at 10 kPa, a first step and asecond step were alternately repeated 120 times. In each of the firststeps, while a Group III source gas (TMG flow rate: 5 sccm) was flown, acarrier gas (mixture of N₂ and H₂, flow rate: 50 slm), a Group V sourcegas (NH₃, flow rate: 15 slm), and a CP₂Mg gas (flow rate: 50 sccm) weresupplied for 15 seconds (supply time t₁). In each of the subsequentsecond steps, only the flow rate of the Group III source gas was changedto a TMG flow rate of 20 sccm, and the Group III source gas, the carriergas, the Group V source gas, and the CP₂Mg gas were supplied for 60seconds (supply time t₂). Thus, a p-GaN layer having a thickness of 1080nm was formed. Note that in the first step, initial growth nuclei weregrown, but a layer was not grown. The crystal growth rate in the secondstep was 0.15 nm/s. The computational growth rate corresponding to theGroup III source gas flow rate in the first step was 0.02 nm/s.

Example 7

In Example 7, after an AlN template substrate having a strain bufferlayer was placed in a growth furnace shown in FIG. 1 and FIG. 2 and thetemperature was increased to 1050° C. at 10 kPa, a first step and asecond step were alternately repeated 120 times. In each of the firststeps, while Group III source gases (TMG flow rate: 2 sccm, TMA flowrate: 5 sccm) were flown, a carrier gas (mixture of N₂ and H₂, flowrate: 50 slm), a Group V source gas (NH₃, flow rate: 15 slm), and aCP₂Mg gas (flow rate: 50 sccm) were supplied for 15 seconds (supply timet₁). In each of the subsequent second steps, only the flow rates of theGroup III source gases were changed to a TMG flow rate of 20 sccm and aTMA flow rate of 45 sccm, and the Group III source gases, the carriergas, the Group V source gas, and the CP₂Mg gas were supplied for 60seconds (supply time t₂). Thus, a p-Al_(0.36)Ga_(0.64)N layer having athickness of 1080 nm was formed. Note that in the first step, initialgrowth nuclei were grown, but a layer was not grown. The crystal growthrate in the second step was 0.15 nm/s. The computational growth ratecorresponding to the Group III source gas flow rate in the first stepwas 0.02 nm/s.

Example 8

In Example 8 , a p-Al_(0.43)Ga_(0.57)N layer having a thickness of 1080nm was formed by a similar method to Example 7 except for the following:in each of the first steps, while Group III source gases (TMG flow rate:2 sccm, TMA flow rate: 6 sccm) were flown, a carrier gas (mixture of N₂and H₂, flow rate: 50 slm), a Group V source gas (NH₃, flow rate: 15slm) and a CP₂Mg gas (flow rate: 50 sccm) were supplied for 15 seconds(supply time t₁); in each of the subsequent second steps, only the flowrates of the Group III source gases were changed to a TMG flow rate of20 sccm and a TMA flow rate of 65 sccm, and the Group III source gases,the carrier gas, the Group V source gas, and the CP₂Mg gas were suppliedfor 60 seconds (supply time t₂); and the first step and the second stepwere alternately repeated. Note that in the first step, initial growthnuclei were grown, but a layer was not grown. The crystal growth rate inthe second step was 0.15 nm/s. The computational growth ratecorresponding to the Group III source gas flow rate in the first stepwas 0.02 nm/s.

Comparative Example 2

In Comparative Example 2, a p-GaN layer having a thickness of 1080 nmwas formed by a similar method to Example 6 except for that the supplytime t₁ was 0 second, the supply time t₂ was 7200 seconds, and therepeat count was one.

Comparative Example 3

In Comparative Example 3, a p-Al_(0.23)Ga_(0.77)N layer having athickness of 1080 nm was formed by a similar method to Example 1 exceptfor that the supply time t₁ was 0 second, the supply time t₂ was 7200seconds, and the repeat count was one.

Comparative Example 4

In Comparative Example 4, a p-Al_(0.36)Ga_(0.64)N layer having athickness of 1080 nm was formed by a similar method to Example 7 exceptfor that the supply time t₁ was 0 second, the supply time t₂ was 7200seconds, and the repeat count was one.

Comparative Example 5

In Comparative Example 5, a p-Al_(0.43)Ga_(0.57)layer having a thicknessof 1080 nm was formed by a similar method to Example 8 except for thatthe supply time t₁ was 0 second, the supply time t₂ was 7200 seconds,and the repeat count was one.

Example 9

As shown in FIG. 6, a superlattice strain buffer layer (AlN/GaN,thickness: 600 nm), an n-Al_(0.23)Ga_(0.77)N layer (thickness: 1300 nm),a light emitting layer (AlInGaN, thickness: 150 nm), ap-Al_(0.36)Ga_(0.64)N blocking layer (thickness: 20 nm), ap-Al_(0.23)Ga_(0.77)N cladding layer (thickness: 180 nm), and a p-GaNcontact layer (thickness: 20 nm) were grown on an AlN template substratehaving an AlN strain buffer layer on a sapphire substrate by MOCVDprocess to produce a Group III nitride semiconductor light emittingdevice.

Here, the p-Al_(0.36)Ga_(0.64)N blocking layer was formed by a similarmethod to Example 7 except for that the supply time t₁ was 15 seconds,the supply time t₂ was 45 seconds, and the repeat count was three.

Example 10

In Example 10, as shown in FIG. 6, a superlattice strain buffer layer(AlN/GaN, thickness: 600 nm), an n-Al_(0.23)Ga_(0.77)N layer (thickness:1300 nm), a light emitting layer (AlInGaN, thickness: 150 nm), ap-Al_(0.43)Ga_(0.57)N blocking layer (thickness: 20 nm), ap-Al_(0.23)Ga_(0.77)N cladding layer (thickness: 180 nm), and a p-GaNcontact layer (thickness: 20 nm) were grown on an AlN template substratehaving an AlN strain buffer layer on a sapphire substrate by MOCVDprocess to produce a Group III nitride semiconductor light emittingdevice.

Here, the p-Al_(0.43)Ga_(0.57)N blocking layer was formed by a similarmethod to Example 8 except for that the supply time t₁ was 10 seconds,the supply time t₂ was 45 seconds, and the repeat count was three.

Comparative Example 6

In Comparative Example 6, a Group III nitride semiconductor lightemitting device having a p-Al_(0.36)Ga_(0.44)N blocking layer wasproduced by a similar method to Example 9 except for that the supplytime t₁ was 0 second, the supply time t₂ was 135 seconds, and the repeatcount was one.

Comparative Example 7

In Comparative Example 7, a Group III nitride semiconductor lightemitting device having a p-Al_(0.43)Ga_(0.57)N blocking layer wasproduced by a similar method to Example 10 except for that the supplytime t₁ was 0 second, the supply time t₂ was 135 seconds, and the repeatcount was one.

(Evaluation 2)

The results of measuring the magnesium concentration of the p-AlGaNblocking layers in the light emitting devices of Example 9 andComparative Example 6 using a SIMS (secondary ion mass spectrometer) areshown in FIG. 7 and FIG. 8, respectively.

Further, as in Evaluation 1 , the carrier concentrations were calculatedfrom the specific resistance of the p-AlGaN single film layers. Theresults are shown in Table 3 and FIG. 9.

TABLE 3 Carrier Total concentration Supply Supply Thickness thicknesscalculated p-AlGaN Al time time Repeat per of single Specific fromSpecific single film composition t1 t2 count r repetition film layerresistance resistance layer ratio (second) (second) (number) (nm) (nm)(W × cm) (/cm3) Example 6 0 15 60 120 9 1080 0.113 1.10 ′ 1019 Example 10.23 15 60 120 9 1080 1.7 7.35 ′ 1017 Example 7 0.36 15 60 120 9 10802.76 4.52 ′ 1017 Example 8 0.43 15 60 120 9 1080 4.47 2.79 ′ 1017Comparative 0 0 7200 1 1080 1080 0.146 8.57 ′ 1018 Example 2 Comparative0.23 0 7200 1 1080 1080 2.75 4.53 ′ 1017 Example 3 Comparative 0.36 07200 1 1080 1080 3.62 3.45 ′ 1017 Example 4 Comparative 0.43 0 7200 11080 1080 5.42 2.30 ′ 1017 Example 5

Table 3 shows that the magnesium concentrations in Examples 6, 1, 7, and8 in accordance with the present invention are higher than those ofComparative Examples 2, 3, 4, and 5 involving the same Al compositions,respectively. This also leads to increase in the effective carrierconcentration, which consequently reduces the specific resistance.

Evaluation 3

Further, the emission EL outputs of back side of the light emittingdevices of the above Examples 9, 10, and Comparative Examples 6 and 7were measured using a multichannel spectrometer (C10082CAH manufacturedby Hamamatsu Photonics K.K.). The results are shown in Table 4.

TABLE 4 p-AlGaN blocking Total layer Supply Supply Thickness thicknessin Light Al time time Repeat per of single emitting composition t1 t2count r repetition film layer EL output device ratio (second) (second)(number) (nm) (nm) (mW) Example 9 0.36 15 45 3 6.67 20 33.1 Example 100.43 10 45 3 6.67 20 25 Comparative 0.36 0 135 1 20 20 15.5 Example 6Comparative 0.43 0 135 1 20 20 6.9 Example 7

Table 4 shows that the EL output of Example 9 in accordance with thepresent invention is significantly improved as compared with ComparativeExample 6. Further, the effect of improved output can also be confirmedin Example 10 involving a higher Al composition ratio as compared withComparative Example 7. These results are considered due to theimprovement in energization accompanying the increase in the carrierconcentration as apparent from Table 4.

INDUSTRIAL APPLICABILITY

According to the present invention, a p-AlGaN layer having a carrierconcentration and a light output which are improved by forming onep-AlGaN layer doped with magnesium using MOCVD under conditions where aGroup III source gas is supplied in a first step at a flow rate of 0 orat a flow rate equal to or less than a flow rate of a Group III sourcegas supplied in a second step can be provided. The present invention canalso provide a method of manufacturing the same and a Group III nitridesemiconductor light emitting device.

Further, the present invention can provide a p-AlGaN layer achieving acarrier concentration and a light output which are improved by repeatingthe first step and the second step a plurality of times, a method ofmanufacturing the same, and a Group III nitride semiconductor lightemitting device.

EXPLANATION OF REFERENCE NUMERALS

-   100: MOCVD system-   101: First gas supply port-   102: Second gas supply port-   103: Growth furnace-   104: Hydrogen gas-   105: Nitrogen gas-   106: TMA-   107: TMG-   108: CP₂Mg-   109: Ammonia-   110: Susceptor-   111: Base substrate-   112: AlGaN layer-   200: Group III nitride semiconductor light emitting device-   201: Base substrate-   202: AlN strain buffer layer-   203: Superlattice strain buffer layer-   204: N-nitride semiconductor layer-   205: Light emitting layer-   206: P-AlGaN blocking layer-   207: P-AlGaN guide layer-   208: P-AlGaN cladding layer-   209: P-GaN contact layer

The invention claimed is:
 1. A method of manufacturing a p-AlGaN layer,the p-AlGaN layer being one p-Al_(x)Ga_(1-x)N layer (0≦x<1) doped withmagnesium, which is formed by MOCVD, comprising the steps of: a firststep of supplying a Group V source gas at a Group V source gas flow rateB₁ (0<B₁) and supplying a gas containing magnesium at a Mg-containinggas flow rate C₁ (0<C₁) while supplying a Group III source gas at aGroup III source gas flow rate A₃ (0<A₃); and a second step of supplyinga Group V source gas at a Group V source gas flow rate B₂ (0<B₂) andsupplying a gas containing magnesium at a Mg-containing gas flow rate C₂(0<C₂) while supplying a Group III source gas at a Group III source gasflow rate A₂ (0<A₂), wherein the first step and the second step areperformed to form the p-Al_(x)Ga_(1-x)N layer, and the Group III sourcegas flow rate A₃ is a flow rate which allows only initial growth nucleiof the p-Al_(x)Ga_(1-x)N layer to grow and satisfies A₃≦0.5A₂.
 2. Amethod of manufacturing a p-AlGaN layer, the p-AlGaN layer being onep-Al_(x)Ga_(1-x)N layer (0≦x<1) doped with magnesium, which is formed byMOCVD, comprising the steps of: a first step of supplying a Group Vsource gas at a Group V source gas flow rate B₁ (0<B₁) and supplying agas containing magnesium at a Mg-containing gas flow rate C₁ (0<C₁)while supplying a Group III source gas at a Group III source gas flowrate A₃ (0<A₃); and a second step of supplying a Group V source gas at aGroup V source gas flow rate B₂ (0<B₂) and supplying a gas containingmagnesium at a Mg-containing gas flow rate C₂ (0<C₂) while supplying aGroup III source gas at a Group III source gas flow rate A₂ (0<A₂),wherein the first step and the second step are repeated a plurality oftimes to form the p-Al_(x)Ga_(1-x)N layer, and the Group III source gasflow rate A₃ is a flow rate which allows only initial growth nuclei ofthe p-Al_(x)Ga_(1-x)N layer to grow and satisfies A₃≦0.5A₂.
 3. Themethod of manufacturing a p-AlGaN layer according to claim 1, whereinthe Group V source gas flow rate B₁ in the first step is equal to theGroup V source gas flow rate B₂ in the second step; and/or theMg-containing gas flow rate C₁ in the first step is equal to theMg-containing gas flow rate C₂ in the second step.
 4. The method ofmanufacturing a p-AlGaN layer according to claim 1, wherein, when arelationship between the Group III source gas flow rate and a crystalgrowth rate is evaluated from the crystal growth rate in the secondstep, the Group III source gas flow rate in the first step is a flowrate such that a growth rate of the p-Al_(x)Ga_(1-x)N layercorresponding to the flow rate is 0.03 nm/s or less.
 5. The method ofmanufacturing a p-AlGaN layer according to claim 1, wherein the aluminumcomposition ratio x of the p-Al_(x)Ga_(1-x)N layer is in a range of 0 to0.8.
 6. The method of manufacturing a p-AlGaN layer according to claim2, wherein the Group V source gas flow rate B₁ in the first step isequal to the Group V source gas flow rate B₂ in the second step; and/orthe Mg-containing gas flow rate C₁ in the first step is equal to theMg-containing gas flow rate C₂ in the second step.
 7. The method ofmanufacturing a p-AlGaN layer according to claim 2, wherein, when arelationship between the Group III source gas flow rate and a crystalgrowth rate is evaluated from the crystal growth rate in the secondstep, the Group III source gas flow rate in the first step is a flowrate such that a growth rate of the p-Al_(x)Ga_(1-x)N layercorresponding to the flow rate is 0.03 nm/s or less.
 8. The method ofmanufacturing a p-AlGaN layer according to claim 2, wherein the aluminumcomposition ratio x of the p-Al_(x)Ga_(1-x)N layer is in a range of 0 to0.8.
 9. The method of manufacturing a p-AlGaN layer according to claim1, wherein the Group III source gas flow rate A₂ is a flow rate whichallows the initial growth nuclei of the p-Al_(x)Ga_(1-x)N layer to growand form a substantial layer.
 10. The method of manufacturing a p-AlGaNlayer according to claim 2, wherein the Group III source gas flow rateA₂ is a flow rate which allows the initial growth nuclei of thep-Al_(x)Ga_(1-x)N layer to grow and form a substantial layer.